This invention relates to methods and apparatus for locating objects such as buried pipes, electrical cables and the like.
It is well known to locate underground objects such as pipes and electrical cables by detecting the magnetic field produced by currents flowing in such objects. If the object is accessible at some point, an RF source may be coupled to the object to excite an alternating current flow therein. The object may then be tracked using locating apparatus that responds to the frequency of the RF source by detecting the magnetic field produced by the current flow. Another convenient source of excitation current is propagating VLF electromagnetic energy such as produced by high-powered radio transmitters operating in the 14-30 KHz frequency band. Such propagating electromagnetic energy penetrates deeply below the earth's surface and will induce current flow in buried objects such as pipes or electrical cables. By detecting the magnetic fields associated with such current flow using apparatus tuned to the frequency of the propagating electromagnetic energy, the buried objects may be located.
Known methods and apparatus for locating buried objects by detecting the magnetic fields associated therewith are subject to error because of extraneous ambient fields. Known locating apparatus typically comprises two or more spaced magnetic field sensors for detecting the magnetic field at spaced locations. As the sensors pass over the buried object, the presence of the object is indicated by changes in the detected magnetic field at each sensor. However, extraneous ambient magnetic fields may produce similar changes in the magnetic fields at the sensors and cause errors in detecting the location of the buried object.
It is desirable, therefore, to provide methods and apparatus for locating current-carrying objects that avoid such problems, and it is to this end that the present invention is directed.